The invention concerns vehicle suspensions. In particular, the invention relates to an axle connecting two wheels, while performing an antiroll function (also called anticamber function). The invention still more specifically relates to the category of axles that embody two suspension arms, one end of which is intended to support a wheel and the other end of which is hinged on the body of the vehicle, and that further embody a cross member connecting the two suspension arms.
Several variants of these axles are known. The cross member is sometimes mounted on the axis coupling the arms on the body, other times it is mounted in an intermediate position between the spindle axis and the axis coupling the arms on the body, and still other times it is mounted on the spindle axis, or slightly beyond the spindles, whether or not crossing a line intersecting the spindle axis and intersecting the axis coupling the arms on the body, notably, depending on the elastokinematic characteristics it is desired to impart to the axle. This type of axle is commonly found at the rear end of passenger cars. It is known that the position of the cross member, that is, its distance from the axis coupling the arms on the body, determines the kinematics of camber and wheel alignment, that is, the variation of camber and wheel alignment as a function of the roll angle. With initial camber and dis-alignment there is no kinematic variation of camber and wheel alignment if the relative axis of rotation of the arms in relation to each other (that is, in general, the axis of the cross member) is in the axis of coupling on the body. Therefore, the handling of the vehicle is influenced by the position of the cross member.
Independent wheel suspensions are not considered here, sometimes presenting in their trailing arm variants a sort of perfectly rigid cross member, that is, indeformable under the effect of working stresses. Such a cross member is always placed on the axis coupling the arms on the body, and the arms are rotatably mounted on the cross member. Such a cross member does not affect the antiroll characteristics of the suspension and the axle in question cannot be described as torsional.
The invention concerns torsion axles, that is, deformable axles, and whose deformation or, in general, the torsional stress of whose cross member contributes to the characteristics of antiroll resistance of the wheel assembly. In that case, the cross member, taken as a whole, undergoes a relative rotation of its axial ends on a transverse axis.
Such a cross member is dimensioned to be very rigid on bending. It helps strictly maintain the plane of the wheel when the suspension arm in turn is subjected to bending and torsional stress by the crosswise transfers of loads. Such stresses are due to the transverse adherence of the tire on the road and can become very considerable on turns negotiated at high speed. In other words, the cross member helps prevent undesirable variation of camber of the wheel or steering of the wheel or at least assists in strictly controlling them, so that they will remain within acceptable limits, or so that they will be controlled and not suffered. Such a cross member, if designed with the sole aim of correctly containing wheel steering and wheel camber, has too high a torsional rigidity. This is why excess torsional rigidity in general is dealt with by adopting an open-section cross member.
Another solution is known for rendering such a cross member less rigid on torsion, while maintaining its bending strength at a high level. U.S. Pat. No. 4,787,680 can be considered in this connection. Unfortunately, such a design is satisfactory only when a sufficient space is available for installing the specific area of the cross member whose section is suited to reducing the torsional rigidity. In fact, the installation of suspension arms and their joints on the body requires a transverse space practically independent of the size of the vehicle. Hence, the space available for said specific area diminishes with the track of the vehicle much faster than proportionally.
In other applications in common use, the cross member is formed by an open section of lower torsional rigidity. It is to be observed, however, that the linkage of such a cross member to the arms raises numerous problems of endurance. The linkage zone is the seat of a high stress concentration, which leads to reinforcing it, for example, by welding additional coupling plates, or by increasing inertia at the end of the cross member. In that case, the part of the cross member really used to control the roll is reduced to a portion roughly lying between the plates or additional reinforcing elements.
That is why the control of the plane of the wheels (geometric aspect) and control of body movements (flexibility aspect, wheel clearance as a function of transfers of loads) are very often treated separately. An antiroll bar separate from the suspension arms guiding and controlling the plane of the wheel steering gear very commonly endows the axle with a roll resistance added to that coming from the suspension springs and to that coming from a cross member rigidly coupled to the suspension arms.
It is observed in the present state of the art that the choice between independent wheels and torsion axle raises some ill-resolved difficulties. It is difficult to compromise the different demands between torsion and bending characteristics.
If it is decided to adopt a wheel assembly of the type with torsion axle rather than with independent wheels, the design of such an axle must satisfy rather contradictory requirements. It is necessary to endow the axle with sufficient bending strength, to achieve a good maintenance of wheel planes, in order to avoid steering of the wheels that is too great or occurring in an undesirable direction on severe transverse stresses. But it is necessary at the same time for the wheel arms to be able to clear one another relatively independently, while preferably having an elastic return to the position where the arms are parallel to each other. This is the antiroll function characteristic of the axle mentioned above. In most known torsion axle solutions, the torsional cross member, which is rigid on bending, is rigidly fixed to the arms, for example, by welding, in order to ensure holding of the wheels.
This type of torsion axle should offer, for small sedans, for example, sufficient bending strength (minimum 45000 mN/rad/m for an arm 250 mm long between the axis of coupling on the body and the axis of the spindle under convergent stresses) combined with a sufficient capacity for torsional elastic deformation (resistance between 250 and 500 mN/rad/m for an arm of the length indicated above), the values being only indicative and depending, in fact, on the vehicle, the height of its center of gravity, the track and the type of handling that the designer wishes to impart to the vehicle.
These types of solutions present great difficulties with regard to the compromise and adjustment between bending strength and torsional rigidity. In fact, the stress concentrations in the housing between cross member and arms require, in order to maintain them at a tolerable level, not only lengthening the arms, thus diminishing the torsional stress angles of the cross member, but also strengthening the housing with added reinforcements. The latter artificially increase torsional rigidity of the section.